Currently, commercial services utilizing satellite navigation systems are expanding considerably. Products operating on the basis of radiolocation signals have become widely accessible in everyday home routine within motor vehicles for aiding road navigation initially and more recently within mobile telephone devices for a multitude of personal services. Upgrades to future satellite positioning systems, for example the GALILEO European system, promise much higher performance than current systems. Thus new services which were not able to be envisaged for lack of sufficient reliability and positioning precision are today conceivable for companies, notably road transport and air transport companies. For example, for road transport, efforts are under way to transform the economic models of the services for operating toll road sections by offering the customer personalized offers. For air transport companies, increased performance in terms of reliability and positioning precision allows the integration within aircraft of navigation devices on which pilots will be able to rely entirely. These devices will make it possible to considerably improve air transport safety. However, for services on which people's safety depends, it is mandatory to prove the reliability of the data sent by the positioning system. This is why operators of satellite navigation systems are made subject by the authorities to requirements regarding guaranteed service to the end customer.
Satellite navigation systems are characterized by performance data relating to integrity, precision and coverage. Integrity is a measure of confidence in the information provided by the satellite positioning system. A well known tool for determining the integrity of a point provided is the Stanford chart. The Stanford chart is a two-dimensional matrix whose input parameter on the horizontal axis is the observed position error vertically or horizontally and whose input parameter on the vertical axis is the protection level vertically or horizontally calculated on the basis of statistical models. This chart makes it possible to verify the proportion of measured samples whose observed position error is lower than the protection level.
The precision of a position is defined by the position error estimated with respect to the actual position. The precision of the location depends notably on the error in the estimated distance between the user and the satellites received as well as on the configuration of the geometry of the measurements. There exists a value, commonly called the DOP for “Dilution of Precision”, which is indicative of the conditions of geometry of the measurements. When the value of the DOP is high, this indicates that the satellites used to obtain the position are close and therefore that the geometry is bad and when the value of the DOP is low this indicates that the satellites used to obtain the position are distant and therefore that the geometry is good.
The bodies responsible for regulations and checks relating to civil aviation require rigorous levels of performance notably as regards precision performance for critical services. Among these critical services utilizing the geo-location data of satellite positioning systems, the LPV200 service (“Localizer Performance with Vertical Guidance”) requires that the satellite positioning system show for at least 95% of the time a location error vertically of less than 4 meters and horizontally of less than 16 meters. Moreover, this service requires that the probability of providing a location error vertically of greater than 10 meters be less than 10−7 under normal conditions and that the probability of providing a location error of greater than 15 meters be less than 10−5 under deteriorated conditions. This service is associated with an alert level vertically at 35 meters and horizontally at 40 meters.
The certified precision levels have been obtained by measuring samples under known sampling conditions, notably as regards measurement geometry. Now, no tool for measuring precision currently exists which allows these levels of requirement to be certified to the user since the precision performance also depends on the satellite geometry, and the data collected do not take into account all cases of satellite geometry for each user.
A solution implemented by the service operator of the American satellite positioning system for certifying a level of precision to each user and taking into account all the configurations of satellite geometry has been to collect data over a period of long duration. Satellite data have been collected for three years in order to be able to assert that all cases of satellite geometry have been observed. However, this scheme presents the disadvantage of mobilizing significant resources, and moreover does not guarantee that it has been possible to observe all cases.